Application of non-invasive brain imaging techniques to the study of neuro-transmitter receptors make it necessary to develop animal models to detect in vivo modulation of receptor number and affinity. The modulation of Dopamine receptors in vivo was studied on rats injecting 3H spiroperidol. In rats chronically treated with typical (haloperidol) and atypical (sulpiride) neuroleptics the number of 3H spiroperidol binding sites was increased. The chronic neuroleptic induced increase in 3H spiroperidol binding was selectively antagonized by the D2 receptor antagonist sulpiride but not by the D1 receptor antagonist SCH-23390. These data suggest that with the use of radiolabeled spiroperidol it is possible to study in vivo the modulation of the kinetics parameters of D1 and D2 Dopamine receptors. To study the GABA receptors we used 3H muscimol injected in mice. Muscimol labels in a saturable manner and high affinity GABA receptor. Diazepan, 1mg/kg increases the Bmax of this binding. Since Diazepan potentiates the pharmacological responses of Muscimol these data suggest that the facilitatory action of Benzodiazepine on GABAergic transmission is mediated by an increase availability of GABA recognition site at the membrane site.